1. Field of the Invention
The present invention generally relates to capacitors having roller electrode sheets and, in particular, to such capacitors which use polypropylene film as a dielectric spacer between the roller electrode sheets.
2. Statement of the Prior Art
Alternating current capacitors having rolled electrode sheets are well known and widely used. Such capacitors generally include a pair of electrode sheets interleaved with a pair dielectric spacer means, with the entire combination being cylindrically rolled. Manufacturers of such capacitors usually experiment with various materials for use in constructing such capacitors for such purposes as increasing capacity, improving space efficiency, improving capacitor reliability and thereby reducing overall capacitor cost. One approach towards these goals is to use polypropylene film as the dielectric spacer between the electrodes. Earlier capacitors typically used paper as the dielectric spacer. Although paper is still used, it does not have a very high breakdown voltage for preventing arcing between the electrodes, thus lowering volumetric efficiency by requiring both greater amounts of material for higher operating voltages and also larger electrode area due to increased electrode spacing. Polypropylene film, on the other hand, enjoys a high dielectric constant and a very high breakdown voltage, which promotes volumetric efficiency for the capacitor along with larger capacitance values for capacitors of similar size.
In the area of reliability, a major factor affecting performance is the arcing or corona discharge between electrodes within the capacitor roll due to the presence of air. The presence of air allows oxidation, the products of which cause interelectrode arcing. Manufacturers have attempted to use various means for reducing or eliminating the presence of air within the capacitor roll. One common method involves impregnating the capacitor roll with oil and then heat shrinking the roll to squeeze out as much of the oil as possible along with any air still remaining. Unfortunately, this approach has not been been completely successful with polypropylene film dielectric spacers. This is because polypropylene film is not easily wetted by many of the various known dielectric fluids. This inhibits penetration of the capacitor roll by dielectric fluid along with its purging of trapped air therefrom. One approach which claims to remedy some of these problems is disclosed in U.S. Pat. No. 3,363,156. This patent discloses a special dielectric fluid along with a dielectric spacer including sheets of capacitor grade paper interleaved with sheets of polypropylene film. The paper is used to wick the special dielectric fluid into the capacitor roll for the purpose of covering the polypropylene film. Unfortunately, this approach still suffers from a certain amount of volumetric inefficiency due to the presence of the paper which separates the electrodes more than they need to be.